Owner's Problem Description
The owner describes a 1992 Saturn SC2 that runs fine for about 10 minutes each morning. After that, it starts cutting out severely while driving, often stalling for 30 seconds before resuming operation. The owner has replaced the coolant temperature sensor, coil packs, plugs, wires, and crank position sensor, but the problem persists. The check engine light does not illuminate, indicating no active faults as the owner has tested the light by unplugging a sensor.
Professional Technical Analysis
The vehicle in question is a 1992 Saturn SC2 with a 1.9L I4 engine, likely accumulating around 150,000 miles. The symptoms of cutting out after warming up indicate a potential issue with the ignition or fuel system, particularly as the vehicle initially runs fine for a short period. The replacement of the coolant temperature sensor, coil packs, spark plugs, wires, and crank position sensor suggests that the owner is already following a logical diagnostic path. However, the lack of check engine light activation complicates the diagnosis. A detailed examination is required of the electrical connections, particularly at the coolant temperature sensor, as corrosion can lead to intermittent failures, especially when the vehicle reaches operating temperature. The PCM (Powertrain Control Module) may also be affected by voltage fluctuations in the charging system, which can impact ignition timing and fuel delivery. It's critical to verify if the ignition control module (ICM) is functioning correctly, as thermal breakdown in these units is common in older Saturn models. Additionally, the fuel pressure regulator should be tested; it should deliver a steady pressure of 35-45 PSI under load. If the pressure drops, it may indicate a failing fuel pump or clogged filter. A comprehensive scan using the GeekOBD APP to monitor live data during operation may reveal fluctuations in these parameters, highlighting the root cause of the stalling issue.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Control Module (ICM) thermal failure: The ICM may fail when hot, which is a common issue in older Saturn models. When the engine warms up, the ICM may experience thermal breakdown, causing intermittent ignition failure. This can lead to the engine cutting out during operation, especially after prolonged running times. It's critical to check the resistance of the ICM under operational temperature and to confirm its integrity with a multimeter. - No check engine light, intermittent stalling after warming up, potential stored codes related to ignition issues.
- Fuel System Pressure Drop: If the fuel pump is failing or the fuel filter is clogged, it may not maintain proper fuel pressure (should be 35-45 PSI). This drop can cause the engine to stutter or stall, particularly after it warms up and the demand for fuel increases. Testing the fuel pressure at the fuel rail during idle and under load conditions is critical to determining the root cause. - Poor acceleration, potential lean running condition, fluctuations in fuel pressure readings.
- Electrical Connector Corrosion: The connector at the coolant temperature sensor could be corroded, leading to poor electrical contact. This may cause incorrect temperature readings, which can mislead the PCM in fuel and timing adjustments. Inspecting and cleaning this connector is crucial, as it could be the root cause of the intermittent stalling. - Visual inspection may show corrosion, intermittent readings from the coolant temperature sensor.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Scan: Begin with a comprehensive scan using the GeekOBD APP to retrieve any stored codes and monitor live data. Pay special attention to the coolant temperature sensor readings, as they should reflect accurate engine temperature. Any fluctuations or incorrect readings could indicate a faulty sensor or wiring issue.
- STEP 2 - Fuel Pressure Testing: Use a fuel pressure gauge to check the pressure at the fuel rail. The pressure should be between 35-45 PSI. If the pressure is lower, inspect the fuel pump and filter for potential issues. This test can reveal if the stalling is related to inadequate fuel supply during operation.
- STEP 3 - Inspect Electrical Connectors: Examine the connector at the coolant temperature sensor for signs of corrosion or damage. Clean any corrosion and ensure a tight electrical connection. This step is vital, as poor connections can lead to intermittent issues that mimic sensor failures.
- STEP 4 - ICM Testing: Test the ignition control module under load using a multimeter. Measure the resistance and check for proper operation. If the ICM fails when hot, it will need to be replaced. This is particularly important for older models like the Saturn SC2, where ICM failure is common.
